1. Field of the Invention
The present invention relates to a radio frequency module operating at a frequency of several hundred MHz or higher and a method for fabricating the same. More particularly, the present invention relates to a radio frequency module including active elements (such as transmission power transistors) mounted on a printed circuit board and a method for fabricating the same.
2. Description of the Related Art
A typical "radio frequency module" implements a compact radio frequency circuit by mounting principally various chip components at a high density on a printed circuit board of a small size. Specifically, a radio frequency module includes various active elements such as transmission power transistors and various kinds of small-sized passive elements or chip components such as resistors, capacitors and inductors. Both the active elements and the passive elements are mounted on a printed circuit board on which transmission lines such as microstrip lines have been formed. A radio frequency module operates at a frequency of several hundred MHz or higher and is typically used as a transmission section for various kinds of mobile communication units such as transceivers, cellular phone units and the like.
A radio frequency module used as a transmission section generally implements a two-stage or three-stage radio frequency power amplifier circuit including a plurality of field effect transistors (hereinafter, simply referred to as "FETs"). The impedance of a radio frequency module viewed from an external circuit at the terminal thereof for inputting/outputting a radio frequency signal therethrough is adjusted to be 50 .OMEGA. and a direct current bias circuit is incorporated therein.
A prior art metal base substrate for a radio frequency module is described, for example, in Japanese Laid-Open Patent Publication No. 3-272189. The metal base substrate described in this patent publication includes an insulating film, and a metal plate and a sheet of conductive metal foil which are adhered to both surfaces of the insulating film.
Also, a prior art radio frequency module is described, for example, in Japanese Laid-Open Patent Publication No. 5-95236. FIG. 13 is a schematic cross-sectional view of the radio frequency module described in this patent publication. The radio frequency module shown in FIG. 13 includes a circuit board 1310 and a semiconductor device 1320 which have been mounted on a radiator plate 1300 by means of soldering. The circuit board 1310 has a multi-layer structure of a metal film 1312, a resin substrate 1314 and another metal film 1316. A solder layer 1330 connects the circuit board 1310 and the semiconductor device 1320 to the radiator plate 1300, respectively.
The semiconductor device 1320 includes: an FET chip 1322; a package metal base 1323; a ceramic frame 1324; electrodes 1325; a source wire 1326; a gate wire 1327; a drain wire 1328; and lead terminals 1329.
The heat generated from the FET chip 1322 is passed through the package metal base 1323 and the solder layer 1330 to be transmitted into the radiator plate 1300.
In order to fabricate the radio frequency module shown in FIG. 13, the circuit board 1310 and the semiconductor device 1320 must be individually soldered to the radiator plate 1300. Thus, the radio frequency module shown in FIG. 13 has a problem in that the number of required process steps is increased. Also, the lead terminals 1329 must be connected to the circuit board 1310 of an individual module.
Furthermore, the radio frequency module shown in FIG. 13 cannot be tested until the radiator plate 1300 has been mounted. This is because the characteristics of the FET chip 1322 in an operation state are required to be tested under the same conditions as the conditions where the radio frequency module is actually used or the state where the radiator plate 1300 has already been mounted. However, in order to mount the radiator plate 1300 onto the radio frequency module, the circuit board 1310 having a small area corresponding to an individual radio frequency module must be cut out from a circuit board having a larger area corresponding to a plurality of radio frequency modules. Consequently, a prior art radio frequency module cannot be tested before the circuit board 1310 for an individual radio frequency module is cut out from the larger sized circuit board. Thus, the radio frequency module can be tested only after the individual radio frequency module is completed. Therefore, in accordance with the conventional technologies, multiple radio frequency modules on an identical large-sized circuit board 1310 cannot be tested simultaneously.
As described above, since the conventional radio frequency module cannot reduce the number of necessary components and the number of necessary process steps, the reduction in size and cost of a single module and the automatic fabrication of the modules cannot be realized.